Airborne particles can be removed from a polluted air stream by a variety of physical processes. Common types of equipment for collecting fine particulates include, for example, cyclones, scrubbers, electrostatic precipitators, and baghouse filters.
Most air-pollution control projects are unique. Accordingly, the type of particle collection device, or combination of devices, to be employed normally must be carefully chosen in each implementation on a case-by-case basis. Important particulate characteristics that influence the selection of collection devices include corrosivity, reactivity, shape, density, and size and size distribution, including the range of different particle sizes in the air stream. Other design factors include air stream characteristics (e.g., pressure, temperature, and viscosity), flow rate, removal efficiency requirements, and allowable resistance to airflow. In general, cyclone collectors are often used to control industrial dust emissions and as precleaners for other collection devices. Wet scrubbers are usually applied in the control of flammable or explosive dusts or mists from such sources as industrial and chemical processing facilities and hazardous-waste incinerators; they can handle hot air streams and sticky particles. Large scale electrostatic precipitators or filtration devices and fabric-filter baghouses are often used at power plants.
Electrostatic precipitation or filtration, which are interchangeable terms, is a commonly used method for removing fine particulates from air streams. In an electrostatic precipitator, an electric charge is imparted to particles suspended in an air stream, which are then removed by the influence of an electric field. A typical precipitation unit or device includes baffles for distributing airflow, discharge and collection electrodes, a dust clean-out system, and collection hoppers. A high DC voltage, often as much as 100,000 volts in large scale applications, is applied to the discharge electrodes to charge the particles, which then are attracted to oppositely charged collection electrodes, on which they become trapped.
In a typical large-scale electrostatic precipitator the collection electrodes consists of a group of large rectangular metal plates suspended vertically and parallel to each other inside a boxlike structure. There are often hundreds of plates having a combined surface area of tens of thousands of square meters. Rows of discharge electrode wires hang between the collection plates. The wires are given a negative electric charge, whereas the places are grounded and thus become positively charged.
Particles that stick to the collection plates are removed periodical iv when the plates are shaken, or “rapped.” Rapping is a mechanical technique for separating the trapped particles from the plates, which typically become covered with a 6-mm (0.2-inch) layer of dust. Rappers are either of the impulse (single-blow) or vibrating type. The dislodged particles are collected in a hopper at the bottom of the unit and removed for disposal. An electrostatic precipitator can remove exceptionally small particulates on the order of 1 micrometer (0.0004 inch) with an efficiency exceeding 99 percent. The effectiveness of electrostatic precipitators in removing fly ash from the combustion gases of fossil-fuel furnaces accounts for their high frequency of use at power stations.
Large-scale electrostatic precipitators are expensive, difficult to build, and quite large. However, electrostatic filtration is exceedingly efficient and highly reliable. As a result, skilled artisans have devoted considerable effort and resources toward the development of small-scale electrostatic precipitators or air filtration devices specifically adapted for small scale applications, such as for filtering breathing. Although considerable attention has been directed toward the development of small-scale and portable electrostatic filtration devices utilized principally to filter breathing air, existing implementations are difficult to construct, expensive, must be constructed to strict and often unattainable tolerances, and cannot be tuned or calibrated as needed to meet specific and/or changing environmental conditions or air filtering requirements. Given these and other deficiencies in the art of electrostatic air filters, the need for continued improvement is evident.